Vascular development results in a stable pattern of blood vessels that can be modified only by specific physiological or pathological signals. Blood vessel formation is based on endothelial cell responses to these signals and other cell-cell interactions of endothelial cells that are mediated through receptor molecules. Tyrosine kinase receptors that bind angiogenic factors, and endothelial cell surface adhesion receptors have functions consistent with a role in blood vessel formation in vivo. The specific involvement of these molecules in developmental and pathological blood vessel formation, however, has not been demonstrated. The experiments described in this proposal will test the hypothesis that endothelial cell receptors are important in blood vessel formation. The role of known receptors in developmental and pathological blood vessel formation will be investigated, and novel receptors that may have a role in blood vessel formation will be identified using murine models that we have developed: cultured mouse endothelial cells and a unique in vitro developmental system using mouse embryonic stem cells. The regulation of receptor expression will be investigated in a mouse endothelial cell line that responds to cytokine stimulation and undergoes morphological alterations, and this regulation will be compared to that of embryonic endothelial cells. Mouse embryonic stem cells that differentiate in vitro will be used to study the role of the known endothelial cell adhesion receptors and the effects of genetic perturbations of tyrosine kinase receptors on vascular development. The endothelial cell cultures will be used to generate cDNA libraries and to isolate novel receptor genes that may be unique to vascular development. Finally, the relationship between vascular development and hematopoiesis will be examined both in terms of lineage and function using the embryonic stem cell differentiation system. The results of these experiments will help to determine the role of endothelial cell receptors in the regulation of blood vessel formation, and with these mouse models both addition and deletion of genetic material by targeted manipulation of endogenous genes will be available as tools for further studies of vasculogenesis and angiogenesis. A molecular understanding of blood vessel formation will also help to develop therapies that target tumor angiogenesis and vascular diseases such as atherosclerosis.